Re: [Qemu-devel] [PATCH V7 3/5] docs: add pvrdma device documentation.

[Michael S. Tsirkin]

On Sun, Jan 14, 2018 at 11:01:45AM +0200, Marcel Apfelbaum wrote:
> +5. Limitations
> +==============

Limitations are fine but need to cause init failures since users
don't poke in the internal documentation.

> +- The device obviously is limited by the Guest Linux Driver features 
> implementation
> +  of the VMware device API.
> +- Memory registration mechanism requires mremap for every page in the buffer 
> in order
> +  to map it to a contiguous virtual address range. Since this is not the 
> data path
> +  it should not matter much.

Depends on the size of the region I guess. Did you try this with several
gigabytes of memory? If we are talking seconds of downtime,
it's worth documenting so people aren't surprised.
Alternatively, limit the max size of MR?

> +- The device requires target page size to be the same as the host page size.

Pls add code to fail init when this is not the case.

> +- QEMU cannot map guest RAM from a file descriptor if a pvrdma device is 
> attached,
> +  so it can't work with huge pages. The limitation will be addressed in the 
> future,
> +  however QEMU allocates Guest RAM with MADV_HUGEPAGE so if there are enough 
> huge
> +  pages available, QEMU will use them.

Same here.

> +- As previously stated, migration is not supported yet, however with some 
> hardware
> +  support can be done.

I don't see a migration blocker.



-- 
MST